A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the downstream direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 135, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
Communication systems, as depicted in FIG. 1, often include amplifiers to boost signal levels, and automatic gain control (AGC) circuits within the amplifier are typically used to monitor and control the gain of such amplifiers.
FIG. 2 is an electrical block diagram of a conventional amplifier 125 that includes an AGC circuit. In operation, the amplifier 125 receives a forward signal from the upstream path at an input port 205. In conventional cable television systems, the forward signals being transmitted have been predominately analog channels. The forward signal is transmitted through one or more gain stages 210 for amplifying the forward signal. The amplified signal is then transmitted through a Bode circuit 215 that varies the signal level by attenuation. A final output gain stage 220 subsequently processes the forward signal, which is then transmitted to an output port 225. The output of the final gain stage 220 is also coupled to an AGC circuit 230 that is used to further control the attenuation of the Bode circuit 215 in response to the signal level of the amplified forward signal.
FIG. 3 is an electrical block diagram of the conventional AGC circuit 230 of FIG. 2. The AGC circuit 230 includes an input port for receiving the forward signal, which is also coupled to the amplifier output 225 of the final gain stage 220. The AGC circuit 230 includes a band pass filter 305 and a gain stage 310 for filtering and amplifying, respectively, the pilot signal. The filtered signal is then transmitted to an AM detector 315 that demodulates the signal to recover an analog video signal. Next, the demodulated analog video signal is amplified through a video amplifier 320.
A video peak detector 325 then samples the horizontal synchronization pulses of the demodulated video signal in order to establish a DC voltage that represents the peak carrier level. The peak detector 325 monitors the DC voltage of the horizontal synchronization pulses of the demodulated video signal to set and hold peak voltage values and to generate a peak voltage signal representative of the peak voltage values. This is generally done by storing the DC voltage values in a capacitor-resistor network between synchronization pulses.
The DC voltage signal provided by the peak detector 325 is compared with a thermal reference level by the integrator 330 to verify that the gain of the amplifier gain stages has remained constant. If the compared peak voltage signal has dropped below or risen above the predetermined thermal reference level, a thermal reference voltage signal from the reference voltage circuit 335 will be provided at the output of the AGC circuit 230 until such time as the voltage level of the pilot carrier signal level again equals the reference voltage level.
Again referring to the forward input signal to the AGC circuit 230, the input signal is filtered through the band pass filter 305 to allow a predetermined pilot carrier signal to pass. The pilot carrier signal is then demodulated and used, through comparison with the reference voltage level, to control the Bode circuit 215 (FIG. 2). The frequency of the pilot carrier signal is generally determined by selecting a median point between the lowest channel and the highest channel in the forward frequency spectrum, although the frequency of the pilot carrier may vary as long as the band pass filter 305 and other device components are configured to process a carrier signal of the desired frequency.
Historically, cable television systems have transmitted only analog signals, so transmission and processing of an analog pilot carrier signal by conventional analog AGC circuits has worked well. The cable television industry, however, is migrating to transmission of digital signals, so a pilot carrier signal in the digital frequency spectrum may, in the future, be chosen for processing through the AGC circuit. These digital signals are generally QAM modulated, and QAM modulated digital signals cannot be accurately processed by prior art AGC circuits, such as the AGC circuit 230 shown in FIG. 3. More specifically, the DC voltage values of the QAM modulated signals include complex, multi-level data having peak values at varying times and different rates, so peak detection in an AGC circuit does not provide useful or accurate information.
Thus, what is needed is an AGC circuit that can process a pilot carrier signal in the digital frequency spectrum to accurately generate a signal level control.